CODEWITHSUNDEEP
arraysfortranoperator-overloading
Thomas Kühn
Thomas Kühn

Reputation: 9820

Problem with un-allocated assumed-shape array in Fortran user-defined operator

I am trying to implement a python-type in operator that checks whether a 1D array contains a certain element. In principle I got this to work, but I'm having trouble covering both types of arrays that I want to work with, namely fixed-size arrays and allocatable arrays. Below I have a code that almost does what I want:

MODULE operator_in
  IMPLICIT NONE

  INTERFACE OPERATOR(.IN.)
     MODULE PROCEDURE in_integer_list
!     MODULE PROCEDURE in_integer_list_alloc
  END INTERFACE OPERATOR(.IN.)

CONTAINS
  FUNCTION in_integer_list(key, list) RESULT(res)
    IMPLICIT NONE
    INTEGER, INTENT(IN) :: key
    INTEGER, INTENT(IN) :: list(:)
    LOGICAL             :: res

    INTEGER :: ii

    res = .FALSE.
    DO ii = 1,SIZE(list)
       IF (key == list(ii)) THEN
          res = .TRUE.
          RETURN
       END IF
    END DO
  END FUNCTION in_integer_list

  FUNCTION in_integer_list_alloc(key, list) RESULT(res)
    IMPLICIT NONE
    INTEGER,              INTENT(IN) :: key
    INTEGER, ALLOCATABLE, INTENT(IN) :: list(:)
    LOGICAL                          :: res

    IF (ALLOCATED(list)) THEN
       res = in_integer_list(key, list)
    ELSE
       res = .FALSE.
    END IF
  END FUNCTION in_integer_list_alloc

END MODULE operator_in


PROGRAM test

  USE operator_in

  INTEGER              :: list1(5) = (/1, 4, 6, 3, 8/)
  INTEGER, ALLOCATABLE :: list2(:), list3(:)
  INTEGER              :: ii


  ALLOCATE(list2(7))
  list2(:) = (/8,7,6,5,4,2,1/)

  DO ii = 1,5
     IF (ii .IN. list1) THEN
        WRITE (*,'(I3,A,5I3)') ii, ' is in ', list1
     END IF
     IF (ii .IN. list2) THEN
        WRITE (*,'(I0.3,A,7I3)') ii, ' is in ', list2
     END IF
!     IF (ii .IN. list3) THEN
!        WRITE (*,'(I3,A,7I3)') ii, ' is in ', list3
!     END IF
  END DO

END PROGRAM test

As is, the code produces the following output:

  1 is in   1  4  6  3  8
  1 is in   8  7  6  5  4  2  1
  2 is in   8  7  6  5  4  2  1
  3 is in   1  4  6  3  8
  4 is in   1  4  6  3  8
  4 is in   8  7  6  5  4  2  1
  5 is in   8  7  6  5  4  2  1

However, if I un-comment the last three lines,

     IF (ii .IN. list3) THEN
        WRITE (*,'(I0.3,A,7I3)') ii, ' is in ', list3
     END IF

the code crashes with a segmentation fault, because list3 is not allocated:

Program received signal SIGSEGV: Segmentation fault - invalid memory reference.

Backtrace for this error:
#0  0x10925ebe4
#1  0x10925e306
#2  0x7fff5e878b5c
#3  0x1092547da
#4  0x109254bc5
#5  0x109254cce
Segmentation fault: 11

I tried to fix this by writing a second function (in_integer_list_alloc) that allows for allocatable arrays, but declaring both functions in my interface:

  INTERFACE OPERATOR(.IN.)
     MODULE PROCEDURE in_integer_list
     MODULE PROCEDURE in_integer_list_alloc
  END INTERFACE OPERATOR(.IN.)

gives me an ambiguity error:

   FUNCTION in_integer_list(key, list) RESULT(res)
  1
user-defined_operator.f90:27:2:

   FUNCTION in_integer_list_alloc(key, list) RESULT(res)
  2
Error: Ambiguous interfaces in operator interface 'in' for 'in_integer_list' at (1) and 'in_integer_list_alloc' at (2)

And if I comment out the first procedure in the interface:

  INTERFACE OPERATOR(.IN.)
!     MODULE PROCEDURE in_integer_list
     MODULE PROCEDURE in_integer_list_alloc
  END INTERFACE OPERATOR(.IN.)

I of course get problems with the fixed-size array, list1:

      IF (ii .IN. list1) THEN
                1
Error: Operands of user operator 'in' at (1) are INTEGER(4)/INTEGER(4)

So: Is there a clever way to achieve what I want or at least get a proper error message when the code crashes because the passed array is not allocated?

Upvotes: 3

Views: 409

Answers (2)

Matt P
Matt P

Reputation: 2357

NB: This answer has a flaw, as pointed out in the comments. I will leave it here so that other readers might avoid making the same mistake.

Being a Python user, I can also appreciate the syntactic sugar that an .in. operator could provide. That said, standard Fortran is also quite concise: 

if (any(items==value)) then
    ...
endif

However, here is one way to implement .in. that should handle both fixed-size and allocatable arrays:

module operator_in
    implicit none
    interface operator(.in.)
        module procedure operatorin
    end interface operator(.in.)

    contains
    logical function operatorin(v,lst) result(found)
        implicit none
        integer, intent(in) :: v
        integer, intent(in) :: lst(:)
        integer, allocatable :: temp(:)

        found = .false.
        allocate(temp,source=lst)
        if (size(temp)>0) then
            if (any(temp == v)) found=.true.
        endif
    end function operatorin
end module operator_in

Notes: I use the allocatable temp array so that if compiled with check:all (or similar) the code will run without error messages. Also, I use any to avoid manually looping over the list of items. Try it out, it's nice.

Upvotes: 1

IanH
IanH

Reputation: 21431

Ideally, redesign your code such that there is no need to deal with an unallocated array. If you want to represent an empty list, use a allocated zero size array.

(An unallocated object is a better conceptual fit to "there is no list", rather than an "the list is empty". Conceptually, you should not be querying something that doesn't exist.)

If you must, you could write a single argument adapter function along the line of the following:

FUNCTION foo(arg)
  INTEGER, INTENT(IN), OPTIONAL:: arg(:)
  INTEGER, ALLOCATABLE :: foo(:)
  IF (PRESENT(arg)) THEN
    foo = arg
  ELSE
    foo = [INTEGER ::]
  END IF
END FUNCTION foo

The adapter could then be used:

 IF (item .in. foo(list)) THEN
   ...

Appropriate naming of the adapter function is left to the reader.

(The adapter has been written with the dummy argument OPTIONAL, to accommodate actual argument not present, actual argument not allocated and actual argument dissociated. This is a Fortran 2008 feature.)

Upvotes: 3

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